An Evolved Universal Mobile Telecommunication System Territorial Radio Access Network (E-UTRAN) and an Evolved Packet Core (EPC) make up an Evolved Packet System (EPS), where the Universal Mobile Telecommunication System is abbreviated to UMTS. Some operators expect to provide voice services still through the traditional 2G/3G radio access network and Circuit-Switched (CS) core network after deploying the EPS network. The EPS network provides only Packet-Switched (PS) data service access. In the foreseeable future, the EPS network will coexist with the traditional 2G/3G CS network. Most User Equipments (UEs) keep a radio connection of one radio access technology at one moment to reduce costs and battery consumption and save air interface signaling. Therefore, a CS fallback solution is put forward. A UE can access the CS core network through a 2G/3G access network such as the Global System for Mobile Communications (GSM)/GSM Enhanced Data rate for GSM Evolution Radio Access Network (GERAN) (Enhanced Data rate for GSM Evolution is abbreviated to EDGE), UTRAN, and Code Division Multiple Access (CDMA) Access Network (AN), or access the EPC through an E-UTRAN. After the UE is attached to the EPS, an association is set up between a Mobility Management Entity (MME) which the UE registers to and a Mobile Switching Center (MSC) in the CS domain which the UE registers to. The interface between the MME and the MSC is defined as an SGs interface in the EPS standard. After the SGs interface association is set up, the UE may access the EPC on the E-UTRAN to use packet services when no CS voice service is in progress. If a voice service needs to be performed, the UE may fall back to the GERAN/UTRAN/CDMA AN through a CS fallback procedure to set up a CS call link according to the existing CS procedure and perform the voice service, which is known as CS fallback. When the UE performs the voice service, the packet bearer of the UE is generally suspended by the network. After the packet bearer is suspended, the network still reserves the IP address of the packet bearer allocated to the UE, but the packet bearer is unable to transmit packet data at this time, namely, the data service is interrupted. After the voice service of the UE is completed, the network may resume the packet bearer of the UE through a resume procedure, and the UE resumes the packet service.
It is assumed that the UE accesses the CS core network through a UTRAN. After the MME is associated with the MSC at the SGs interface (the setup of the SGs interface association means that the UE accesses the EPC through an E-UTRAN currently and registers with the MSC through the EPC), if the MSC receives a UE Initial Address Message (IAM) from the calling party, the MSC forwards a paging request message to the MME. After the MME receives the paging request message, if the UE is currently connected, the MME sends a paging notification message to the UE directly through a signaling connection between the MME and the UE. After receiving the paging notification message, the UE may display prompt information according to the CS fallback configuration and the current service status on the UE. The prompt information indicates to the user that a voice call arrives and asks the user whether to interrupt the current data service and fall back to the CS domain to answer the voice call. If the UE receives a command input by the user intending to interrupt the current data service and answer the voice call, the UE may notify the MME to perform a CS fallback procedure, fall back to the CS domain and set up a CS call link for the voice service, and hand over the current packet bearer to the UTRAN and suspend it. If the UE receives a command input by the user intending to reject the voice call, the UE may send a service request message carrying the cause of rejection to the MME. After receiving the message, the MME notifies the MSC that the called user rejects the voice call through an SGs interface. The MSC responds to the calling party according to a paging failure procedure or a UE rejection procedure. In this case, the ongoing UE data service on the E-UTRAN is not affected.
In the process of implementing the present invention, the inventor finds at least these problems in the prior art: After the UE displays the prompt information, the user of the UE needs to confirm whether to “interrupt the current data service and answer the voice call” or “continue the current data service and reject the voice call”. The confirmation is a man-machine interaction process. It takes a few seconds or more for the user of the UE to see the prompt information, make a judgment, and send confirmation information to the UE. After the MSC sends a paging request message to the MME, the MSC keeps waiting for a paging response from the UE. Generally, the waiting time of the MSC is over 10 seconds. In the CS fallback solution, if the MSC fails to receive the paging response from the UE in a long period because of waiting for the confirmation of the CS fallback operation from the user, the MSC is unable to know whether the UE failed to be paged on the E-UTRAN side, or whether the UE is still waiting for the confirmation from the user after successfully receives the paging request on the E-UTRAN side. Consequently, the MSC mistakenly believes that the paging fails, and terminates the call procedure or attempts the paging on the 2G/3G access network instead mistakenly. In the existing CS domain, after the called user is paged successfully and a CS call link is set up, an alerting message is returned to the calling party, and the calling party hears the ring back tone, which takes only a few seconds normally. In the CS fallback solution, however, while waiting for the called user to confirm the CS fallback operation, the calling party may terminate the call actively because the calling party hears no ring back tone in a long period, and mistakenly believes that the network fails. Thus the voice service is affected.